Solid Electrolytic Capacitor Containing an Improved Manganese Oxide Electrolyte

ABSTRACT

A solid electrolytic capacitor that contains an anode body formed from an electrically conductive powder and a dielectric coating located over and/or within the anode body is provided. The powder has a high specific charge and in turn a relative dense packing configuration. Despite being formed from such a powder, the present inventors have discovered that a manganese precursor solution (e.g., manganese nitrate) can be readily impregnated into the pores of the anode. This is accomplished, in part, through the use of a dispersant in the precursor solution that helps minimize the likelihood that the manganese oxide precursor will form droplets upon contacting the surface of the dielectric. Instead, the precursor solution can be better dispersed so that the resulting manganese oxide has a “film-like” configuration and coats at least a portion of the anode in a substantially uniform manner. This improves the quality of the resulting oxide as well as its surface coverage, and thereby enhances the electrical performance of the capacitor.

RELATED APPLICATIONS

The present application claims priority to Provisional Application Ser.No. 61/357,672 (filed on Jun. 23, 2010) and Provisional Application Ser.No. 61/366,657 (filed on Jul. 22, 2010), both which are incorporatedherein in their entirety by reference thereto.

BACKGROUND OF THE INVENTION

Manganese dioxide is known and widely used as a solid electrolyte inelectrolyte capacitors. Such capacitors are conventionally formed byfirst anodizing a valve-metal anode (e.g., tantalum) to form adielectric oxide coating, and thereafter immersing the oxide-coatedanode in an aqueous solution of manganese nitrate. After a sufficientperiod of time, the wet anode is heated to cause pyrolytic decompositionof the manganese nitrate to manganese dioxide. To achieve the desiredthickness of the solid electrolyte, the steps of immersion and heatingare often repeated multiple times. Unfortunately, one problem withconventional manganizing techniques is that the thickness of theresulting manganese dioxide is often greater at certain locations of theanode (e.g., edges), which can lead to poor electrical performance.Various techniques have been employed in an attempt to address theseproblems. For example, surfactants have been employed in the manganesenitrate solution to substantially reduce its surface tension and improvethe wettability of the surface of the oxide-coated anode. One suchsurfactant is Erktantol® NR (Tanatex Chemicals BV), which is a nonionicfatty alcohol polyglycol ether. Likewise, U.S. Pat. No. 4,302,301 toTierman describes various other nonionic surfactants that can beemployed in the manganizing solution, such asnonylphenoxypoly-(ethyleneoxy)ethanol (Igepal CO-630);isooctylphenoxy-polyethoxyethanol (Triton X-100),benzyletheroctylphenol-ethylene oxide condensate (Triton CF-10), and3,6-dimethyl-4-octyne-3,6-diol (Surfynol 82).

Although the addition of surfactants may provide some benefits,significant problems nevertheless remain. For example, the capacitorsmay still exhibit a relatively large loss in capacitance when wet and ahigh leakage current. This problem is particularly evident when thevalve metal powder used to form the anode has a high specificcharge—i.e., about 70,000 microFarads*Volts per gram (“μF*V/g”) or more.Such high “CV/g” powders are generally formed from particles having asmall size and large surface area, which results in the formation ofsmall pores between the particles that are difficult to impregnate withthe manganese nitrate solution. The difficulty in impregnating suchsmall pores leads to the formation of manganese dioxide particles thatare large in size and irregularly shaped. These particles do not adherewell to the dielectric coating and are unable to achieve good surfacecoverage, which leads to poor electrical performance of the capacitor.

As such, a need currently exists for an improved electrolytic capacitorcontaining a manganese oxide solid electrolyte.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the present invention, a solidelectrolytic capacitor is disclosed that comprises an anode body formedfrom an electrically conductive powder, wherein the powder has aspecific charge of about 70,000 μF*V/g or more. A dielectric overliesthe anode body. Further, a solid electrolyte overlies the dielectric,wherein the solid electrolyte includes a manganese oxide film that coatsat least a portion of the dielectric in a substantially uniform manner.

In accordance with another embodiment of the present invention, a methodfor forming a solid electrolytic capacitor is disclosed. The methodcomprises anodically oxidizing an anode body to form a dielectriccoating, wherein the anode body is formed from a powder. Thedielectric-coated anode body is contacted with a manganese oxideprecursor solution that contains a dispersant. In one embodiment, thedispersant includes an organic compound having a hydrophilic moiety anda hydrophobic moiety, which is an aromatic or heteroatomic ring systemhaving from 6 to 14 carbon atoms. In another embodiment, the ratio ofthe surface tension of water (at 20° C.) to the surface tension of thedispersant (at a concentration of 1 wt. % in water and at 20° C.) isfrom about 0.8 to about 1.2. The precursor is pyrolytically converted toa manganese oxide solid electrolyte.

Other features and aspects of the present invention are set forth ingreater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

A full and enabling disclosure of the present invention, including thebest mode thereof, directed to one of ordinary skill in the art, is setforth more particularly in the remainder of the specification, whichmakes reference to the appended figures in which:

FIG. 1 is a schematic illustration of one embodiment of a capacitor thatmay be formed in accordance with the present invention;

FIGS. 2-4 are FESEM photographs of the samples of Example 1, whereinFIG. 2 is a photograph of Sample 1, FIG. 3 is a photograph of Sample 2,and FIG. 4 is a photograph of Sample 3;

FIGS. 5-7 are FESEM photographs of the samples of Example 2, whereinFIG. 5 is a photograph of Sample 1, FIG. 6 is a photograph of Sample 2,and FIG. 7 is a photograph of Sample 3;

FIGS. 8-10 are FESEM photographs of the samples of Example 3, whereinFIG. 8 is a photograph of Sample 1, FIG. 9 is a photograph of Sample 2,and FIG. 10 is a photograph of Sample 3;

FIGS. 11-12 are FESEM photographs of the samples of Example 4, whereinFIG. 11 is a photograph of Sample 1 and FIG. 12 is a photograph ofSample 2; and

FIG. 13 shows the particle size distribution (number of particles versusthe hydrodynamic diameter) for the solutions formed in Example 5.

Repeat use of references characters in the present specification anddrawings is intended to represent same or analogous features or elementsof the invention.

DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS

It is to be understood by one of ordinary skill in the art that thepresent discussion is a description of exemplary embodiments only, andis not intended as limiting the broader aspects of the presentinvention, which broader aspects are embodied in the exemplaryconstruction.

Generally speaking, the present invention is directed to a solidelectrolytic capacitor that contains an anode body formed from anelectrically conductive powder and a dielectric coating located overand/or within the anode body. The powder may have a high specific chargeand in turn a relative dense packing configuration. Despite being formedfrom such a powder, the present inventors have discovered that amanganese precursor solution (e.g., manganese nitrate) can be readilyimpregnated into the pores of the anode. This is accomplished, in part,through the use of a dispersant in the precursor solution that helpsminimize the likelihood that the manganese oxide precursor will formdroplets upon contacting the surface of the dielectric. Instead, theprecursor solution can be better dispersed so that the resultingmanganese oxide has a “film-like” configuration and coats at least aportion of the anode in a substantially uniform manner. This improvesthe quality of the resulting oxide as well as its surface coverage, andthereby enhances the electrical performance of the capacitor. Variousembodiments of the invention will now be described in more detail.

I. Anode

As indicated above, the anode may be formed from a powder having a highspecific charge. That is, the powder may have a specific charge of about70,000 microFarads*Volts per gram (“μF*V/g”) or more, in someembodiments about 80,000 μF*V/g or more, in some embodiments about90,000 μF*V/g or more, in some embodiments about 100,000 μF*V/g or more,and in some embodiments, from about 120,000 to about 250,000 μF*V/g. Ofcourse, although powders of a high specific charge are normally desired,it is not necessarily a requirement. In certain embodiments, forexample, powders having a specific charge of less than about 70,000microFarads*Volts per gram (“μF*V/g”), in some embodiments about 2,000μF*V/g to about 65,000 μF*V/g, and in some embodiments, from about 5,000to about 50,000 μF*V/g.

The powder may contain individual particles and/or agglomerates of suchparticles. Compounds for forming the powder include a valve metal (i.e.,metal that is capable of oxidation) or valve metal-based compound, suchas tantalum, niobium, aluminum, hafnium, titanium, alloys thereof,oxides thereof, nitrides thereof, and so forth. For example, the valvemetal composition may contain an electrically conductive oxide ofniobium, such as niobium oxide having an atomic ratio of niobium tooxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments1:1.0±0.1, and in some embodiments, 1:1.0±0.05. For example, the niobiumoxide may be NbO_(0.7), NbO_(1.0), NbO_(1.1), and NbO₂. Examples of suchvalve metal oxides are described in U.S. Pat. Nos. 6,322,912 to Fife;6,391,275 to Fife et al.; 6,416,730 to Fife et al.; 6,527,937 to Fife;6,576,099 to Kimmel, et al.; 6,592,740 to Fife, et al.; and 6,639,787 toKimmel, et al.; and 7,220,397 to Kimmel, al., as well as U.S. PatentApplication Publication Nos. 2005/0019581 to Schnitter; 2005/0103638 toSchnitter, et al.; 2005/0013765 to Thomas, et al., all of which areincorporated herein in their entirety by reference thereto for allpurposes.

The apparent density (or Scott density) of the powder may vary asdesired, but typically ranges from about 1 to about 8 grams per cubiccentimeter (g/cm³), in some embodiments from about 2 to about 7 g/cm³,and in some embodiments, from about 3 to about 6 g/cm³. To achieve thedesired level of packing and apparent density, the size and shape of theparticles (or agglomerates) may be carefully controlled. For example,the shape of the particles may be generally spherical, nodular, etc. Theparticles may have an average size of from about 0.1 to about 20micrometers, in some embodiments from about 0.5 to about 15 micrometers,and in some embodiments, from about 1 to about 10 micrometers.

The powder may be formed using techniques known to those skilled in theart. A precursor tantalum powder, for instance, may be formed byreducing a tantalum salt (e.g., potassium fluotantalate (K₂TaF₇), sodiumfluotantalate (Na₂TaF₇), tantalum pentachloride (TaCF₅), etc.) with areducing agent (e.g., hydrogen, sodium, potassium, magnesium, calcium,etc.). Such powders may be agglomerated in a variety of ways, such asthrough one or multiple heat treatment steps at a temperature of fromabout 700° C. to about 1400° C., in some embodiments from about 750° C.to about 1200° C., and in some embodiments, from about 800° C. to about1100° C. Heat treatment may occur in an inert or reducing atmosphere.For example, heat treatment may occur in an atmosphere containinghydrogen or a hydrogen-releasing compound (e.g., ammonium chloride,calcium hydride, magnesium hydride, etc.) to partially sinter the powderand decrease the content of impurities (e.g., fluorine). If desired,agglomeration may also be performed in the presence of a gettermaterial, such as magnesium. After thermal treatment, the highlyreactive coarse agglomerates may be passivated by gradual admission ofair. Other suitable agglomeration techniques are also described in U.S.Pat. Nos. 6,576,038 to Rao; 6,238,456 to Wolf, et al.; 5,954,856 toPathare, et al.; 5,082,491 to Rerat; 4,555,268 to Getz; 4,483,819 toAlbrecht, et al.; 4,441,927 to Getz, et al.; and 4,017,302 to Bates, etal., which are incorporated herein in their entirety by referencethereto for all purposes.

The desired size and/or shape of the particles may be achieved bycontrolling various processing parameters as is known in the art, suchas the parameters relating to powder formation (e.g., reduction process)and/or agglomeration (e.g., temperature, atmosphere, etc.). Millingtechniques may also be employed to grind a precursor powder to thedesired size. Any of a variety of milling techniques may be utilized toachieve the desired particle characteristics. For example, the powdermay initially be dispersed in a fluid medium (e.g., ethanol, methanol,fluorinated fluid, etc.) to form a slurry. The slurry may then becombined with a grinding media (e.g., metal balls, such as tantalum) ina mill. The number of grinding media may generally vary depending on thesize of the mill, such as from about 100 to about 2000, and in someembodiments from about 600 to about 1000. The starting powder, the fluidmedium, and grinding media may be combined in any proportion. Forexample, the ratio of the starting powder to the grinding media may befrom about 1:5 to about 1:50. Likewise, the ratio of the volume of thefluid medium to the combined volume of the starting powder may be fromabout 0.5:1 to about 3:1, in some embodiments from about 0.5:1 to about2:1, and in some embodiments, from about 0.5:1 to about 1:1. Someexamples of mills that may be used in the present invention aredescribed in U.S. Pat. Nos. 5,522,558; 5,232,169; 6,126,097; and6,145,765, which are incorporated herein in their entirety by referencethereto for all purposes. Milling may occur for any predetermined amountof time needed to achieve the target size. For example, the milling timemay range from about 30 minutes to about 40 hours, in some embodiments,from about 1 hour to about 20 hours, and in some embodiments, from about5 hours to about 15 hours. Milling may be conducted at any desiredtemperature, including at room temperature or an elevated temperature.After milling, the fluid medium may be separated or removed from thepowder, such as by air-drying, heating, filtering, evaporating, etc.

Various other conventional treatments may also be employed in thepresent invention to improve the properties of the powder. For example,in certain embodiments, the particles may be treated with sinterretardants in the presence of a dopant, such as aqueous acids (e.g.,phosphoric acid). The amount of the dopant added depends in part on thesurface area of the powder, but is typically present in an amount of nomore than about 200 parts per million (“ppm”). The dopant may be addedprior to, during, and/or subsequent to any heat treatment step(s).

The particles may also be subjected to one or more deoxidationtreatments to improve ductility and reduce leakage current in theanodes. For example, the particles may be exposed to a getter material(e.g., magnesium), such as described in U.S. Pat. No. 4,960,471, whichis incorporated herein in its entirety by reference thereto for allpurposes. The getter material may be present in an amount of from about2% to about 6% by weight. The temperature at which deoxidation occursmay vary, but typically ranges from about 700° C. to about 1600° C., insome embodiments from about 750° C. to about 1200° C., and in someembodiments, from about 800° C. to about 1000° C. The total time ofdeoxidation treatment(s) may range from about 20 minutes to about 3hours. Deoxidation also preferably occurs in an inert atmosphere (e.g.,argon). Upon completion of the deoxidation treatment(s), the magnesiumor other getter material typically vaporizes and forms a precipitate onthe cold wall of the furnace. To ensure removal of the getter material,however, the fine agglomerates and/or coarse agglomerates may besubjected to one or more acid leaching steps, such as with nitric acid,hydrofluoric acid, etc.

To facilitate the construction of the anode, certain components may alsobe included in the powder. For example, the powder may be optionallymixed with a binder and/or lubricant to ensure that the particlesadequately adhere to each other when pressed to form the anode body.Suitable binders may include, for instance, poly(vinyl butyral);poly(vinyl acetate); poly(vinyl alcohol); poly(vinyl pyrollidone);cellulosic polymers, such as carboxymethylcellulose, methyl cellulose,ethyl cellulose, hydroxyethyl cellulose, and methylhydroxyethylcellulose; atactic polypropylene, polyethylene; polyethylene glycol(e.g., Carbowax from Dow Chemical Co.); polystyrene,poly(butadiene/styrene); polyamides, polyimides, and polyacrylamides,high molecular weight polyethers; copolymers of ethylene oxide andpropylene oxide; fluoropolymers, such as polytetrafluoroethylene,polyvinylidene fluoride, and fluoro-olefin copolymers; acrylic polymers,such as sodium polyacrylate, poly(lower alkyl acrylates), poly(loweralkyl methacrylates) and copolymers of lower alkyl acrylates andmethacrylates; and fatty acids and waxes, such as stearic and othersoapy fatty acids, vegetable wax, microwaxes (purified paraffins), etc.The binder may be dissolved and dispersed in a solvent. Exemplarysolvents may include water, alcohols, and so forth. When utilized, thepercentage of binders and/or lubricants may vary from about 0.1% toabout 8% by weight of the total mass. It should be understood, however,that binders and/or lubricants are not necessarily required in thepresent invention.

The resulting powder may be compacted to form a pellet using anyconventional powder press device. For example, a press mold may beemployed that is a single station compaction press containing a die andone or multiple punches. Alternatively, anvil-type compaction pressmolds may be used that use only a die and single lower punch. Singlestation compaction press molds are available in several basic types,such as cam, toggle/knuckle and eccentric/crank presses with varyingcapabilities, such as single action, double action, floating die,movable platen, opposed ram, screw, impact, hot pressing, coining orsizing. The powder may be compacted around an anode lead (e.g., tantalumwire). It should be further appreciated that the anode lead mayalternatively be attached (e.g., welded) to the anode body subsequent topressing and/or sintering of the anode body.

After compaction, any binder/lubricant may be removed by heating thepellet under vacuum at a certain temperature (e.g., from about 150° C.to about 500° C.) for several minutes. Alternatively, thebinder/lubricant may also be removed by contacting the pellet with anaqueous solution, such as described in U.S. Pat. No. 6,197,252 toBishop, et al., which is incorporated herein in its entirety byreference thereto for all purposes. Thereafter, the pellet is sinteredto form a porous, integral mass. For example, in one embodiment, thepellet may be sintered at a temperature of from about 1200° C. to about2000° C., and in some embodiments, from about 1500° C. to about 1800° C.under vacuum or an inert atmosphere. Upon sintering, the pellet shrinksdue to the growth of bonds between the particles. The pressed density ofthe pellet after sintering may vary, but is typically from about 2.0 toabout 7.0 grams per cubic centimeter, in some embodiments from about 2.5to about 6.5, and in some embodiments, from about 3.0 to about 6.0 gramsper cubic centimeter. The pressed density is determined by dividing theamount of material by the volume of the pressed pellet.

In addition to the techniques described above, any other technique forconstructing the anode may also be utilized in accordance with thepresent invention, such as described in U.S. Pat. Nos. 4,085,435 toGalvagni; 4,945,452 to Sturmer, et al.; 5,198,968 to Galvagni; 5,357,399to Salisbury; 5,394,295 to Galvagni, et al.; 5,495,386 to Kulkarni; and6,322,912 to Fife, which are incorporated herein in their entirety byreference thereto for all purposes.

Although not required, the thickness of the anode may be selected toimprove the electrical performance of the capacitor. For example, thethickness of the anode may be about 4 millimeters or less, in someembodiments, from about 0.05 to about 2 millimeters, and in someembodiments, from about 0.1 to about 1 millimeter. The shape of theanode may also be selected to improve the electrical properties of theresulting capacitor. For example, the anode may have a shape that iscurved, sinusoidal, rectangular, U-shaped, V-shaped, etc. The anode mayalso have a “fluted” shape in that it contains one or more furrows,grooves, depressions, or indentations to increase the surface to volumeratio to minimize ESR and extend the frequency response of thecapacitance. Such “fluted” anodes are described, for instance, in U.S.Pat. Nos. 6,191,936 to Webber, et al.; 5,949,639 to Maeda, et al.; and3,345,545 to Bourgault et al., as well as U.S. Patent ApplicationPublication No. 2005/0270725 to Hahn, et al., all of which areincorporated herein in their entirety by reference thereto for allpurposes.

II. Dielectric

Once constructed, the anode may be anodized so that a dielectric layeris formed over and/or within the anode. Anodization is anelectrochemical process by which the anode is oxidized to form amaterial having a relatively high dielectric constant. For example, atantalum anode may be anodized to tantalum pentoxide (Ta₂O₅). Typically,anodization is performed by initially applying an electrolyte to theanode, such as by dipping anode into the electrolyte. The electrolyte isgenerally in the form of a liquid, such as a solution (e.g., aqueous ornon-aqueous), dispersion, melt, etc. A solvent is generally employed inthe electrolyte, such as water (e.g., deionized water); ethers (e.g.,diethyl ether and tetrahydrofuran); alcohols (e.g., methanol, ethanol,n-propanol, isopropanol, and butanol); triglycerides; ketones (e.g.,acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g.,ethyl acetate, butyl acetate, diethylene glycol ether acetate, andmethoxypropyl acetate); amides (e.g., dimethylformamide,dimethylacetamide, dimethylcaprylic/capric fatty acid amide andN-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile,butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethylsulfoxide (DMSO) and sulfolane); and so forth. The solvent mayconstitute from about 50 wt. % to about 99.9 wt. %, in some embodimentsfrom about 75 wt. % to about 99 wt. %, and in some embodiments, fromabout 80 wt. % to about 95 wt. % of the electrolyte. Although notnecessarily required, the use of an aqueous solvent (e.g., water) isoften desired to help achieve the desired oxide. In fact, water mayconstitute about 50 wt. % or more, in some embodiments, about 70 wt. %or more, and in some embodiments, about 90 wt. % to 100 wt. % of thesolvent(s) used in the electrolyte.

The electrolyte is ionically conductive and may have an ionicconductivity of about 1 milliSiemens per centimeter (“mS/cm”) or more,in some embodiments about 30 mS/cm or more, and in some embodiments,from about 40 mS/cm to about 100 mS/cm, determined at a temperature of25° C. To enhance the ionic conductivity of the electrolyte, a compoundmay be employed that is capable of dissociating in the solvent to formions. Suitable ionic compounds for this purpose may include, forinstance, acids, such as hydrochloric acid, nitric acid, sulfuric acid,phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.;organic acids, including carboxylic acids, such as acrylic acid,methacrylic acid, malonic acid, succinic acid, salicylic acid,sulfosalicylic acid, adipic acid, maleic acid, malic acid, oleic acid,gallic acid, tartaric acid, citric acid, formic acid, acetic acid,glycolic acid, oxalic acid, propionic acid, phthalic acid, isophthalicacid, glutaric acid, gluconic acid, lactic acid, aspartic acid,glutaminic acid, itaconic acid, trifluoroacetic acid, barbituric acid,cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoic acid,etc.; sulfonic acids, such as methanesulfonic acid, benzenesulfonicacid, toluenesulfonic acid, trifluoromethanesulfonic acid,styrenesulfonic acid, naphthalene disulfonic acid,hydroxybenzenesulfonic acid, dodecylsulfonic acid,dodecylbenzenesulfonic acid, etc.; polymeric acids, such aspoly(acrylic) or poly(methacrylic) acid and copolymers thereof (e.g.,maleic-acrylic, sulfonic-acrylic, and styrene-acrylic copolymers),carageenic acid, carboxymethyl cellulose, alginic acid, etc.; and soforth. The concentration of ionic compounds is selected to achieve thedesired ionic conductivity. For example, an acid (e.g., phosphoric acid)may constitute from about 0.01 wt. % to about 5 wt. %, in someembodiments from about 0.05 wt. % to about 0.8 wt. %, and in someembodiments, from about 0.1 wt. % to about 0.5 wt. % of the electrolyte.If desired, blends of ionic compounds may also be employed in theelectrolyte.

A current is passed through the electrolyte to form the dielectriclayer. The value of voltage manages the thickness of the dielectriclayer. For example, the power supply may be initially set up at agalvanostatic mode until the required voltage is reached. Thereafter,the power supply may be switched to a potentiostatic mode to ensure thatthe desired dielectric thickness is formed over the surface of theanode. Of course, other known methods may also be employed, such aspulse or step potentiostatic methods. The voltage typically ranges fromabout 4 to about 200 V, and in some embodiments, from about 9 to about100 V. During anodic oxidation, the electrolyte can be kept at anelevated temperature, such as about 30° C. or more, in some embodimentsfrom about 40° C. to about 200° C., and in some embodiments, from about50° C. to about 100° C. Anodic oxidation can also be done at ambienttemperature or lower. The resulting dielectric layer may be formed on asurface of the anode and within its pores.

III. Solid Electrolyte

As indicated above, the capacitor of the present invention contains amanganese oxide (e.g., MnO₂) as a solid electrolyte. The manganese oxideis formed through pyrolytic decomposition of a precursor (e.g.,manganese nitrate (Mn(NO₃)₂)), such as described in U.S. Pat. No.4,945,452 to Sturmer, et al., which is incorporated herein in itsentirety by reference thereto for all purposes. For example, adielectric-coated anode body may be contacted with a solution (e.g.,dipped, immersed, sprayed, etc.) that contains the precursor andthereafter heated for conversion into the oxide. If desired, multipleapplication steps may be employed to achieve the desired thickness. Inone embodiment, for example, the anode body is dipped into a firstsolution of a manganese oxide precursor, heated, and then into a secondsolution of manganese oxide precursor and heated. This process may berepeated until the desired thickness is reached.

While the constituents of the manganese oxide precursor solution(s) mayvary in each application step, if multiple steps are employed, it isgenerally desired that at least one of the solutions contains adispersant that is an organic compound containing a hydrophilic moietyand a hydrophobic moiety. The hydrophilic moiety may, for example,include a sulfonate, phosphonate, carboxylate, thiol, sulfonate ester,phosphite, phosphonite, phosphinite, phosphate, sulfate, phosphateester, sulfoxide, sulfone, amino, etc., as well as mixtures and/or saltsthereof. Unlike conventional surfactants, the hydrophobic moiety of thedispersant is generally too small to substantially reduce the surfacetension of the solution. For example, the hydrophobic moiety may be anaromatic or heteroatomic ring system having from 6 to 14 carbon atoms(substituted or unsubstituted), such as benzene, naphthalene,anthracene, toluene, xylene, pyridine, quinoline, isoquinoline,pyrazine, acridine, pyrimidine, pyridazine, etc.

Because the dispersant does not substantially lower the surface tensionof the solution, it may have a surface tension that is approximately thesame as water. For instance, the ratio of the surface tension of water(at 20° C.) to the surface tension of the dispersant (at a concentrationof 1 wt. % in water and at 20° C.) may be from about 0.5 to about 2.0,in some embodiments from about 0.8 to about 1.2, and in someembodiments, from about 0.9 to about 1.1. In certain embodiments, thesurface tension of the dispersant (at a concentration of 1 wt. % inwater and at 20° C.) is from about 50 to about 95 dynes per centimeter,in some embodiments from about 55 to about 80 dynes per centimeter, andin some embodiments, from about 58 to about 68 dynes per centimeter. Thesurface tension of water is about 70 dynes per centimeter. To thecontrary, conventional surfactants typically have a much lower surfacetension. For example, Triton X-100 and Erkantol® NR are believed to bothhave a surface tension of approximately 30 dynes per centimeter (at aconcentration of 1 wt. % in water at 20° C.). As is well known in theart, surface tension can be measured using commercially available forcetensiometers or optical tensiometers (also known as contact angle meteror goniometer) in accordance with ISO 304 (1985), Cor 1:1998) and/orASTM D 1331-89 (Method A).

In one particular embodiment, for example, the dispersant may contain anorganic compound having the following structure, or a salt thereof:

wherein,

R₁ is an alkyl group having from 1 to 6 carbon atoms;

R₂ is a hydrophilic moiety, such as sulfonate, phosphonate, carboxylate,thiol, sulfonate ester, phosphite, phosphonite, phosphinite, phosphate,sulfate, phosphate ester, sulfoxide, sulfone, amino, etc., andcombinations thereof;

m is from 0 to 8, in some embodiments from 0 to 4, and in oneembodiment, 0;

p is from 1 to 8, in some embodiments from 1 to 4, and in oneembodiment, 1; and

n is from 1 to 100, and in some embodiments, from 2 to 30. It should beunderstood that the R₁ and R₂ groups may be bonded to one or more of thecarbon atoms of the ring system. Also, if desired, the compound may bein the form of a salt in which the cation is an alkali metal (e.g.,sodium, potassium, ammonium, etc.), alkaline metal (e.g., calcium),ammonia (NH₄ ⁺), etc. Comparable compounds with a benzene nucleus alsocan be used.

The molecular weight of the dispersant may generally vary as desired,but is typically about 10,000 grams per mole or less, in someembodiments about 6,000 grams per mole or less, and in some embodiments,from about 2,000 to about 5,000 grams per mole. Suitable startingmaterials for forming such dispersants are well known in the art and mayinclude, for instance, naphthalene-α-sulfonic acid (dihydrate),naphthalene-β-sulfonic acid (monohydrate), 2-methylnapthalene-6-sulfonicacid, etc. One particularly suitable dispersant that may be employed inthe present invention is an alkali or alkaline metal salt of a condensednaphthalene sulfonic acid. Such compounds may be prepared as describedin U.S. Pat. No. 3,067,243, the entirety of which is incorporated hereinfor all relevant purposes. For instance, the compound may be prepared bysulfonating naphthalene with sulfuric acid, condensing the sulfonatednaphthalene with formaldehyde, and then neutralizing the condensate soobtained with a base (e.g., sodium hydroxide, potassium hydroxide,calcium hydroxide, etc.). The resulting salt of condensed naphthalenesulfonic acid may have the following structure:

wherein,

R₂ is SO₃;

p is an integer from 1 to 8;

n is from 1 to 100; and

M is sodium, potassium, or calcium. Particularly suitable sodium,potassium, or calcium salts of condensed naphthalene sulfonate arecommercially available under the trade name Daxad® 11 (available fromGeo Specialty Chemicals), Spolostan 4P or 7P (available from Enaspol,a.s., Czech Republic), Proxmat PL-C 753 FP (available from Synthron),and Darvan® 1 (available from R.T. Vanderbilt Co., Inc.

Rather than impacting surface tension, the dispersant of the presentinvention helps “disperse” droplets that initially form when themanganese oxide precursor contacts the surface of the dielectric.Because these droplets become dispersed, the manganese oxide precursoris able to penetrate into very small spaces between the anode particlesto increase the degree of surface coverage. Furthermore, the reductionin droplet formation also allows the coating to assume a film-likeconfiguration that substantially covers a certain area of thedielectric. This improves the quality of the resulting oxide as well asits surface coverage.

In addition, the dispersant may also lead to the formation of acolloidal suspension of nano-sized manganese oxide precursor particles.Without intending to be limited by theory, it is believed that suchnano-sized particles can result in the formation of smaller crystals onthe surface of the anode during the initial application stages of themanganese oxide coating. Such smaller crystals can, in turn, enhance theavailable surface area for subsequent manganese oxide applications. Thismay ultimately result in a coating that is substantially uniform withexcellent surface coverage.

The nano-sized particles may, for instance, have an average diameter ofabout 100 nanometers, about 50 nanometers or less, in some embodimentsfrom about 0.1 to about 30 nanometers, in some embodiments from about0.2 to about 10 nanometers, and in some embodiments, from about 0.4 toabout 2 nanometers. The term “diameter” generally refers to the“hydrodynamic equivalent diameter” of a particle as determined usingknown techniques, such as photon correlation spectroscopy, dynamic lightscattering, quasi-elastic light scattering, etc. These methods aregenerally based on the correlation of particle size with diffusionproperties of particles obtained from Brownian motion measurements.Brownian motion is the random movement of the particles due tobombardment by the solvent molecules that surround the particles. Thelarger the particle, the more slowly the Brownian motion will be.Velocity is defined by the translational diffusion coefficient. Themeasured particle size value thus relates to how the particle moveswithin a liquid and is termed the “hydrodynamic diameter.” Variousparticle size analyzers may be employed to measure the diameter in thismanner. One particular example is a Corouan VASCO 3 Particle SizeAnalyzer. Although not necessarily required, the nano-sized particlesmay also have a narrow particle size distribution, which may furtherimprove the uniformity of the resulting manganese oxide coating. Forinstance, 50% or more, in some embodiments 70% or more, and in someembodiments, 90% or more of the particles may have an average sizewithin the ranges noted above. The number of particles having a certainsize may be determined using the techniques noted above, wherein thepercent volume can be correlated to the number of particles having acertain absorbance unit (“au”).

To achieve the desired improvement in the impregnation of the manganeseoxide precursor without adversely impacting other characteristics of thecapacitor, it is generally desired that the concentration of thedispersant is selectively controlled within a certain range. Forexample, the solution into which the anode body is first dipped maycontain the dispersant in an amount of from about 0.001 wt. % to about 5wt. %, in some embodiments from about 0.005 wt. % to about 2 wt. %, andin some embodiments, from about 0.01 wt. % to about 1 wt. %. Theprecursor(s) (e.g., manganese nitrate) may likewise constitute fromabout 1 wt. % to about 55 wt. % in some embodiments from about 2 wt. %to about 15 wt. %, and in some embodiments, from about 5 wt % to about10 wt. %, of the solution.

A carrier, such as water, is also employed in the solution. Aqueoussolutions of the present invention may, for instance, contain water inan amount of from about 30 wt. % to about 95 wt. %, in some embodimentsfrom about 40 wt. % to about 99 wt. % and in some embodiments, fromabout 50 wt. % to about 95 wt. %. In addition to the components notedabove, the manganese nitrate solution may also contain other additivesthat improve the formation of the resulting oxide. In one embodiment,for example, an alcohol may be used to enhance the wettability of thedielectric with the solution. Suitable alcohols may include, forinstance, methanol, ethanol, n-propanol, isopropanol, butanol, etc., aswell as mixtures thereof. The concentration of the alcohol(s), whenemployed, may be from about 0.1 wt. % to about 50 wt. %, and in someembodiments, from about 0.5 wt. % to about 2 wt. %.

It should be understood that the actual amounts of the components in thesolution may vary depending upon such factors as the particle size anddistribution of particles in the anode, the temperature at whichdecomposition is performed, the identity of the dispersant, the identityof the carrier, the identity of the alcohol, etc. Furthermore, it shouldalso be understood that differing concentrations may be employed indifferent application steps. For example, a first set of one or moredipping steps may be employed in which the manganese oxide precursor ispresent at a first concentration. Thereafter, a second set of one ormore dipping steps may be employed in which the manganese oxideprecursor is present at a second concentration. In some cases, thesecond concentration may be higher than the first concentration.

The amount of time in which the anode body is in contact with themanganese oxide precursor solution may vary as desired. For example, theanode body may be dipped into such a solution for a period of timeranging from about 10 seconds to about 10 minutes. The time may be thesame or different for each individual dipping step. Thedielectric-coated anode body may be at room temperature or pre-driedprior to contact with the precursor solution.

Regardless, once contacted with the precursor solution for the desiredamount of time, the part is heated to a temperature sufficient topyrolytically convert the precursor (e.g., manganese nitrate) to anoxide. Heating may occur, for instance, in a furnace at a temperature offrom about 150° C. to about 300° C., in some embodiments from about 180°C. to about 290° C., and in some embodiments, from about 190° C. toabout 260° C. Heating may be conducted in a moist or dry atmosphere. Thetime for the conversion depends on the furnace temperature, heattransfer rate and atmosphere, but generally is from about 3 to about 5minutes. After pyrolysis, the leakage current may sometimes be high dueto damage suffered by the dielectric film during the deposition of themanganese dioxide. To reduce this leakage, the capacitor may be reformedin an anodization bath as is known in the art. For example, thecapacitor may be dipped into an electrolyte such as described above andthen subjected to a DC current.

IV. Other Components of the Capacitor

If desired, the capacitor may also contain other layers as is known inthe art. For example, a protective coating may optionally be formedbetween the dielectric and solid electrolyte, such as one made of arelatively insulative resinous material (natural or synthetic). Suchmaterials may have a specific resistivity of greater than about 10 Ω/cm,in some embodiments greater than about 100, in some embodiments greaterthan about 1,000 Ω/cm, in some embodiments greater than about 1×10⁵Ω/cm, and in some embodiments, greater than about 1×10¹⁰ Ω/cm. Someresinous materials that may be utilized in the present inventioninclude, but are not limited to, polyurethane, polystyrene, esters ofunsaturated or saturated fatty acids (e.g., glycerides), and so forth.For instance, suitable esters of fatty acids include, but are notlimited to, esters of lauric acid, myristic acid, palmitic acid, stearicacid, eleostearic acid, oleic acid, linoleic acid, linolenic acid,aleuritic acid, shellolic acid, and so forth. These esters of fattyacids have been found particularly useful when used in relativelycomplex combinations to form a “drying oil”, which allows the resultingfilm to rapidly polymerize into a stable layer. Such drying oils mayinclude mono-, di-, and/or tri-glycerides, which have a glycerolbackbone with one, two, and three, respectively, fatty acyl residuesthat are esterified. For instance, some suitable drying oils that may beused include, but are not limited to, olive oil, linseed oil, castoroil, tung oil, soybean oil, and shellac. These and other protectivecoating materials are described in more detail U.S. Pat. No. 6,674,635to Fife, et al., which is incorporated herein in its entirety byreference thereto for all purposes.

If desired, the part may also be applied with a carbon layer (e.g.,graphite) and silver layer, respectively. The silver coating may, forinstance, act as a solderable conductor, contact layer, and/or chargecollector for the capacitor and the carbon coating may limit contact ofthe silver coating with the solid electrolyte. Such coatings may coversome or all of the solid electrolyte.

The capacitor may also be provided with terminations, particularly whenemployed in surface mounting applications. For example, the capacitormay contain an anode termination to which the anode lead of thecapacitor element is electrically connected and a cathode termination towhich the cathode of the capacitor element is electrically connected.Any conductive material may be employed to form the terminations, suchas a conductive metal (e.g., copper, nickel, silver, nickel, zinc, tin,palladium, lead, copper, aluminum, molybdenum, titanium, iron,zirconium, magnesium, and alloys thereof). Particularly suitableconductive metals include, for instance, copper, copper alloys (e.g.,copper-zirconium, copper-magnesium, copper-zinc, or copper-iron),nickel, and nickel alloys (e.g., nickel-iron). The thickness of theterminations is generally selected to minimize the thickness of thecapacitor. For instance, the thickness of the terminations may rangefrom about 0.05 to about 1 millimeter, in some embodiments from about0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2millimeters. One exemplary conductive material is a copper-iron alloymetal plate available from Wieland (Germany). If desired, the surface ofthe terminations may be electroplated with nickel, silver, gold, tin,etc. as is known in the art to ensure that the final part is mountableto the circuit board. In one particular embodiment, both surfaces of theterminations are plated with nickel and silver flashes, respectively,while the mounting surface is also plated with a tin solder layer.

Referring to FIG. 1, one embodiment of an electrolytic capacitor 30 isshown that includes an anode termination 62 and a cathode termination 72in electrical connection with a capacitor element 33. The capacitorelement 33 has an upper surface 37, lower surface 39, front surface 36,and rear surface 38. Although it may be in electrical contact with anyof the surfaces of the capacitor element 33, the cathode termination 72in the illustrated embodiment is in electrical contact with the lowersurface 39 and rear surface 38. More specifically, the cathodetermination 72 contains a first component 73 positioned substantiallyperpendicular to a second component 74. The first component 73 is inelectrical contact and generally parallel with the lower surface 39 ofthe capacitor element 33. The second component 74 is in electricalcontact and generally parallel to the rear surface 38 of the capacitorelement 33. Although depicted as being integral, it should be understoodthat these portions may alternatively be separate pieces that areconnected together, either directly or via an additional conductiveelement (e.g., metal).

The anode termination 62 likewise contains a first component 63positioned substantially perpendicular to a second component 64. Thefirst component 63 is in electrical contact and generally parallel withthe lower surface 39 of the capacitor element 33. The second component64 contains a region 51 that carries an anode lead 16. In theillustrated embodiment, the region 51 possesses a “U-shape” for furtherenhancing surface contact and mechanical stability of the lead 16.

The terminations may be connected to the capacitor element using anytechnique known in the art. In one embodiment, for example, a lead framemay be provided that defines the cathode termination 72 and anodetermination 62. To attach the electrolytic capacitor element 33 to thelead frame, a conductive adhesive may initially be applied to a surfaceof the cathode termination 72. The conductive adhesive may include, forinstance, conductive metal particles contained with a resin composition.The metal particles may be silver, copper, gold, platinum, nickel, zinc,bismuth, etc. The resin composition may include a thermoset resin (e.g.,epoxy resin), curing agent (e.g., acid anhydride), and coupling agent(e.g., silane coupling agents). Suitable conductive adhesives may bedescribed in U.S. Patent Application Publication No. 2006/0038304 toOsako, et al., which is incorporated herein in its entirety by referencethereto for all purposes. Any of a variety of techniques may be used toapply the conductive adhesive to the cathode termination 72. Printingtechniques, for instance, may be employed due to their practical andcost-saving benefits.

A variety of methods may generally be employed to attach theterminations to the capacitor. In one embodiment, for example, thesecond component 64 of the anode termination 62 and the second component74 of the cathode termination 72 are initially bent upward to theposition shown in FIG. 1. Thereafter, the capacitor element 33 ispositioned on the cathode termination 72 so that its lower surface 39contacts the adhesive and the anode lead 16 is received by the upperU-shaped region 51. If desired, an insulating material (not shown), suchas a plastic pad or tape, may be positioned between the lower surface 39of the capacitor element 33 and the first component 63 of the anodetermination 62 to electrically isolate the anode and cathodeterminations.

The anode lead 16 is then electrically connected to the region 51 usingany technique known in the art, such as mechanical welding, laserwelding, conductive adhesives, etc. For example, the anode lead 16 maybe welded to the anode termination 62 using a laser. Lasers generallycontain resonators that include a laser medium capable of releasingphotons by stimulated emission and an energy source that excites theelements of the laser medium. One type of suitable laser is one in whichthe laser medium consist of an aluminum and yttrium garnet (YAG), dopedwith neodymium (Nd). The excited particles are neodymium ions Nd³⁺. Theenergy source may provide continuous energy to the laser medium to emita continuous laser beam or energy discharges to emit a pulsed laserbeam.

Upon electrically connecting the anode lead 16 to the anode termination62, the conductive adhesive may then be cured. For example, a heat pressmay be used to apply heat and pressure to ensure that the electrolyticcapacitor element 33 is adequately adhered to the cathode termination 72by the adhesive. Once the capacitor element is attached, the lead frameis enclosed within a resin casing, which may then be filled with silicaor any other known encapsulating material. The width and length of thecase may vary depending on the intended application. Suitable casingsmay include, for instance, “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “J”,“K”, “L”, “M”, “N”, “P”, “R”, “S”, “T”, “V”, “W”, “Y”, “X”, or “Z” cases(AVX Corporation). Regardless of the case size employed, the capacitorelement is encapsulated so that at least a portion of the anode andcathode terminations are exposed for mounting onto a circuit board. Asshown in FIG. 1, for instance, the capacitor element 33 is encapsulatedin a case 28 so that a portion of the anode termination 62 and a portionof the cathode termination 72 are exposed.

Regardless of the particular manner in which it is formed, the resultingcapacitor may possess a high volumetric efficiency and also exhibitexcellent electrical properties. Even at such high volumetricefficiencies, the equivalent series resistance (“ESR”) may still be lessthan about 200 milliohms, in some embodiments less than about 100milliohms, and in some embodiments, from about 1 to about 50 milliohms,as measured with a 2.2 volt DC bias and a 0.5 volt peak to peaksinusoidal signal, free of harmonics, at a frequency of 100 kHz. Inaddition, the leakage current, which generally refers to the currentflowing from one conductor to an adjacent conductor through aninsulator, can be maintained at relatively low levels. For example, thenumerical value of the normalized leakage current of a capacitor of thepresent invention is, in some embodiments, less than about 1 μA/μF*V, insome embodiments less than about 0.5 μA/μF*V, and in some embodiments,less than about 0.1 μA/μF*V, where μA is microamps and μF*V is theproduct of the capacitance and the rated voltage.

In addition, the capacitor can also exhibit a relatively high percentageof its wet capacitance, which enables it to have only a smallcapacitance loss and/or fluctuation in the presence of atmospherehumidity. This performance characteristic is quantified by the “dry towet capacitance percentage”, which is determined by the equation:

Dry to Wet Capacitance=(1−([Wet−Dry]/Wet))×100

The capacitor of the present invention, for instance, may exhibit a dryto wet capacitance percentage of about 80% or more, in some embodimentsabout 85% or more, in some embodiments about 90% or more, and in someembodiments, from about 92% to 100%.

The capacitance, ESR and normalized leakage current values may even bemaintained after aging for a substantial amount of time. For example,the values may be maintained for about 100 hours or more, in someembodiments from about 300 hours to about 2500 hours, and in someembodiments, from about 400 hours to about 1500 hours (e.g., 500 hours,600 hours, 700 hours, 800 hours, 900 hours, 1000 hours, 1100 hours, or1200 hours) at temperatures ranging from about 20° C. to about 250° C.,and, in some embodiments from about 25° C. to about 100° C. (e.g., 20°C. or 25° C.).

The present invention may be better understood with reference to thefollowing examples.

Test Procedures Equivalent Series Resistance (ESR)

Equivalence series resistance may be measured using a Keithley 3330Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 voltpeak to peak sinusoidal signal. The operating frequency was 100 kHz andthe temperature was 23° C.±2° C.

Dry and Wet Capacitance

The capacitance was measured using a Keithley 3330 Precision LCZ meterwith Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peaksinusoidal signal. The operating frequency was 120 Hz and thetemperature was 23° C.±2° C., The “dry capacitance” refers to thecapacitance of the part after application of the manganese oxide,graphite, and silver layers, while the “wet capacitance” refers to thecapacitance of the part after formation of the dielectric, measured in17% sulfuric acid in reference to 1 mF tantalum cathode.

Leakage Current:

Leakage current (“DCL”) was measured using a leakage test set thatmeasures leakage current at a temperature of 25° C. and at the ratedvoltage after a minimum of 30 seconds.

Life Testing:

For life testing, twenty five (25) samples of the capacitor weresoldered onto a testing plate and put into an oven at the rated voltageof the part and an elevated temperature (e.g., 85° C.) for 2000 hours.

Example 1 150,000 μF*V/g Tantalum Powder (Capacitors 100 μF/4V)

Initially, 3×100,000 capacitor element samples were formed from tantalumanodes having a size of 1.65 mm (length)×1.15 mm (width)×0.85 mm(thickness). Each anode was embedded with a tantalum wire, pressed to adensity of 5.85 g/cm³, and sintered at 1270° C. for 20 minutes. Thetantalum anode was anodized in an orthophosphoric acid/water solutionhaving a conductivity of 8.6 mS/cm and temperature of 85° C. with aforming voltage of 11 volts. The samples were initially dipped into anaqueous solution of manganese(II) nitrate (1150 kg/m³) for 150 secondsand then decomposed at 250° C. to achieve the MnO₂ electrolyte. Thisstep was repeated eight times.

Thereafter, a first set of samples was dipped into an aqueous solutionof manganese(II) nitrate (1300 kg/m³) and 3 g/dm³ of a dispersant(Spolostan 4P, a sodium salt of naphthalene sulfonic acid, polymerizedwith formaldehyde and produced in Enaspol, Czech Republic), and thendecomposed at 250° C. to achieve the MnO₂ cathode. The dispersant had asurface tension of about 62 dynes per centimeter (at a concentration of1 wt. % in water at 20° C.). These steps were repeated six times. Asecond set of the samples was dipped into an aqueous solution ofmanganese(II) nitrate (1300 kg/m³) and 3 g/dm³ of Erkantol NR (TanatexChemicals BV) for 150 seconds then dried at 250° C. to achieve an MnO₂cathode. These steps were repeated six times. Finally, a third set ofsamples was dipped into an aqueous solution of only manganese(II)nitrate (1300 kg/m³) for 150 seconds and decomposed at 250° C. toachieve an MnO₂ cathode. This step was repeated six times. All of thesamples were then dipped into high specific gravity manganese(II)nitrate sequentially into a graphite dispersion and in a silverdispersion and dried.

The median wet capacitance (for 300 samples) was 105 μF.

The finished capacitor elements were completed by conventional assemblytechnology and tested for electrical properties. The results are setforth below.

Median of Electrical Parameters (based on 100,000 parts) Dry to Wet DCL[μA] Dry Capacitance Capacitance Sample (soak time of 70 sec) [μF] [%] 1Spolostan 4P 0.34 87.5 83.3 2 Erkantol NR 0.92 88.2 84.0 3 Control 1.2871.3 67.9 Median of Electrical Parameters Before and after Life Testing(based on 25 parts) DCL [μA] Dry Capacitance (soak time of 70 sec) [μF]Sample Before After Before After 1 Spolostan 4P 0.25 0.25 91.1 85.0 2Erkantol NR 0.64 0.59 96.4 81.1 3 Control 1.23 1.19 72.1 67.2

As indicated, the samples made from the dispersant of the presentinvention (Sample 1) exhibited very low leakage current and a high dryto wet capacitance percentage, even after life testing. FIGS. 2-4 alsoshow FESEM photographs that were taken of the finished capacitors ofSamples 1-3, respectively. As shown in FIG. 2, the sample made fromSpolostan 4P contains manganese dioxide crystals 200 that uniformly coatthe dielectric 204 formed on a tantalum particle 202. Furthermore, themanganese oxide crystals are relatively small and substantiallyhomogenously distributed through the sample. In comparison, the samplesshown in FIGS. 3-4 contain a substantial number of large, unevenlydistributed manganese oxide crystals.

Example 2 70,000 μF*V/g Tantalum Powder (Capacitors 220 μF/6.3V)

Initially, 2×100,000 capacitor element samples were formed from tantalumanodes having a size of 1.8 mm (length)×2.45 mm (width)×1.35 mm(thickness). Each anode was embedded with a tantalum wire, pressed to adensity of 6.1 g/cm³, and sintered at 1295° C. for 20 minutes. Thetantalum anode was anodized in an orthophosphoric acid/water solutionhaving a conductivity of 8.6 mS/cm and temperature of 85° C. with aforming voltage of 9 volts. The samples were dipped into a conventionalaqueous solution of manganese(II) nitrate (1050 kg/m³) for 150 secondsand then decomposed at 250° C. This step was repeated two times. Next,the samples were dipped into a conventional aqueous solution ofmanganese(II) nitrate (1150 kg/m³) for 150 seconds and then decomposedat 250° C. This step was repeated eight times. Thereafter, a first setof samples was dipped into an aqueous solution of manganese(II) nitrate(1300 kg/m³) and 3 g/dm³ of Spolostan 4P and then decomposed at 250° C.This step was repeated eight times. A second set of the samples wasdipped into an aqueous solution of manganese(II) nitrate (1300 kg/m³)and 3 g/dm³ of Erkantol NR for 150 seconds then decomposed at 250° C.This step was repeated eight times. Finally, a third set of samples wasdipped into an aqueous solution of only manganese(II) nitrate (1300kg/m³) for 150 seconds and decomposed at 250° C. This step was repeatedeight times. All of the samples were then dipped into high specificgravity manganese(II) nitrate sequentially into a graphite dispersionand in a silver dispersion and dried.

The median wet capacitance (300 samples) was 236 μF.

The finished capacitor elements were completed by conventional assemblytechnology and tested for electrical properties. The results are setforth below.

Median of Electrical Parameters (based on 100,000 parts) Dry to Wet DCL[μA] Dry Capacitance Capacitance Sample (soak time of 70 sec) [μF] [%] 1Spolostan 4P 1.40 212.2 89.9 2 Erkantol NR 1.88 209.0 88.6 3 Control1.92 196.3 83.2 Median of Electrical Parameters Before and after LifeTesting (based on 25 parts) DCL [μA] Dry Capacitance (soak time of 70sec) [μF] Sample Before After Before After 1 Spolostan 4P 1.18 0.59215.8 204.0 2 Erkantol NR 1.71 1.60 214.0 200.5 3 Control 1.86 1.80193.2 189.5

As indicated, the samples made from the dispersant of the presentinvention (Sample 1) exhibited very low leakage current and a high dryto wet capacitance percentage, even after life testing. FIGS. 5-7 alsoinclude FESEM photographs that were taken of the finished capacitors ofSamples 1-3, respectively. As shown, the sample made from Spolostan 4P(FIG. 5) contains relatively small manganese oxide crystals that aresubstantially homogenously distributed through the sample. Incomparison, the samples shown in FIGS. 6-7 contain a substantial numberof large, unevenly distributed manganese oxide crystals.

Example 3 80,000 μF*V/g Niobium Oxide Powder (Capacitors 220 μF/6.3V)

Initially, 3×20,000 capacitor element samples were formed from NbOanodes having a size of 3.5 mm (length)×2.7 mm (width)×1.65 mm(thickness). Each anode was embedded with a tantalum wire, pressed to adensity of 3.1 g/cm³, and sintered at 1460° C. for 20 minutes. The NbOanodes were anodized in an orthophosphoric acid/water solution having aconductivity of 8.6 mS/cm and temperature of 85° C. with a formingvoltage of 15V. The samples were dipped into a conventional aqueoussolution of manganese(II) nitrate (1150 kg/m³) for 150 seconds and thendecomposed at 200° C. This step was repeated six times. Thereafter, afirst set of samples was dipped into an aqueous solution ofmanganese(II) nitrate (1300 kg/m³) and 3 g/dm³ of Spolostan 4P and thendecomposed at 200° C. This step was repeated two times. A second set ofthe samples was dipped into an aqueous solution of manganese(II) nitrate(1300 kg/m³) and 3 g/dm³ of Erkantol NR for 150 seconds then decomposedat 200° C. This step was repeated two times. Finally, a third set ofsamples was dipped into an aqueous solution of only manganese(II)nitrate (1300 kg/m³) for 150 seconds and decomposed at 200° C. This stepwas repeated two times. All of the samples were then dipped into highspecific gravity manganese(II) nitrate sequentially into a graphitedispersion and in a silver dispersion and dried.

The median wet capacitance (for 60 samples) was 242 μF.

The finished capacitor elements were completed by conventional assemblytechnology and tested for electrical properties. The results are setforth below.

Median of Electrical Parameters (based on 20,000 parts) Dry to Wet DCL[μA] Dry Capacitance Capacitance Sample (soak time of 45 sec) [μF] [%] 1Spolostan 4P 1.76 224.6 92.8 2 Erkantol NR 2.35 214.4 88.6 3 Control2.36 212.5 87.8 Median of Electrical Parameters Before and after LifeTesting (based on 25 parts) DCL [μA] Dry Capacitance (soak time of 45sec) [μF] Sample Before After Before After 1 Spolostan 4P 1.77 15.0221.8 216.5 2 Erkantol NR 2.29 17.2 217.3 214.9 3 Control 2.31 20.1216.0 209.2

As indicated, the samples made from the dispersant of the presentinvention (Sample 1) exhibited very low leakage current and a high dryto wet capacitance percentage, even after life testing. FIGS. 8-10 alsoinclude FESEM photographs that were taken of the finished capacitors ofSamples 1-3, respectively. As shown, the sample made from Spolostan 4P(FIG. 8) contains relatively small manganese oxide crystals that aresubstantially homogenously distributed through the sample, Incomparison, the samples shown in FIGS. 9-10 contain a substantial numberof large, unevenly distributed manganese oxide crystals.

Example 4 18,000 μF*V/g Tantalum Powder (Capacitors 47 μF/35V)

Initially, 2×10,000 capacitor element samples were formed from tantalumanodes having a size of 4.8 mm (length)×3.4 mm (width)×3.1 mm(thickness). Each anode was embedded with a tantalum wire, pressed to adensity of 5.3 g/cm³, and sintered at 1500° C. for 20 minutes. Thetantalum anodes were dipped into an orthophosphoric acid/water solutionhaving a conductivity of 2.9 mS/cm and temperature of 85° C. with aformation voltage of 101 volts. The samples were dipped into aconventional aqueous solution of manganese(II) nitrate (1150 kg/m³) for150 seconds and then decomposed at 250° C. This step was repeated sixtimes. Thereafter, a first set of samples was dipped into an aqueoussolution of manganese(II) nitrate (1300 kg/m³) and 3 g/dm³ of Spolostan4P and then decomposed at 250° C. to achieve the MnO₂ cathode. Thesesteps were repeated two times. A second set of the samples was dippedinto an aqueous solution of only manganese(II) nitrate (1300 kg/m³) for150 seconds and decomposed at 250° C. to achieve an MnO₂ cathode. Thisstep was repeated two times. All of the samples were then dipped intohigh specific gravity manganese(II) nitrate sequentially into a graphitedispersion and in a silver dispersion and dried.

The median wet capacitance (for 30 samples) was 52 μF.

The finished capacitor elements were completed by conventional assemblytechnology and tested for electrical properties. The results are setforth below.

Median of Electrical Parameters (based on 10,000 parts) Dry to Wet DCL[μA] Dry Capacitance Capacitance Sample (soak time of 45 sec) [μF] [%] 1Spolostan 4P 0.44 50.0 96.2 2 Control 0.53 47.1 90.6 Median ofElectrical Parameters Before and after Life Testing (based on 25 parts)DCL [μA] Dry Capacitance (soak time of 45 sec) [μF] Sample Before AfterBefore After 1 Spolostan 4P 0.39 0.20 47.6 47.2 2 Control 0.54 0.20 46.346.0

As indicated, the samples made from the dispersant of the presentinvention (Sample 1) exhibited very low leakage current and a high dryto wet capacitance percentage, even after life testing. FIGS. 11-12 alsoshow FESEM photographs that were taken of the finished capacitors ofSamples 1-2, respectively. As shown, the sample made from Spolostan 4P(FIG. 11) contains relatively small manganese oxide crystals that aresubstantially homogenously distributed through the sample. Incomparison, the samples shown in FIG. 12 contain a substantial number oflarge, unevenly distributed manganese oxide crystals.

Example 5

The ability to form a colloidal suspension of nano-sized precursorparticles was demonstrated. Solutions were initially prepared bydissolving 0.3 g/l of five (5) different dispersants in a conventionalaqueous solution of manganese(II) nitrate (1300 kg/m³) at 20° C. Thedispersants were Daxad® 11 (Geo Specialty Chemicals), Spolostan 7(Enaspol, a.s), Proxmat PL-C 753 FP (Synthron), and Darvan® 1 (R.T.Vanderbilt Co., Inc.). Each of the solutions was allowed to age over one(1) month. The samples were then filtered through a 0.2 μm membranefilter to remove dust and other macro-sized particles before beingsubjected to nanoparticle size analysis using dynamic lightscattering/photon correlation spectroscopy (Cordouan VASCO 3 ParticleSize Analyzer). The results are shown in FIG. 13. As illustrated,colloidal suspensions were formed of various particle sizedistributions. To confirm that the formation of the nano-sized particleswas due to the combination of the dispersant and the nitrate precursor,two (2) control samples were also formed. The first control was anaqueous solution of manganese(II) nitrate (1300 kg/m³) and the secondcontrol was a solution of 0.3 g/l of Spolostan 7 (Enaspol, a.s) inwater. Upon testing, neither of the control solutions was shown tocontain particulates.

These and other modifications and variations of the present inventionmay be practiced by those of ordinary skill in the art, withoutdeparting from the spirit and scope of the present invention. Inaddition, it should be understood that aspects of the variousembodiments may be interchanged both in whole or in part. Furthermore,those of ordinary skill in the art will appreciate that the foregoingdescription is by way of example only, and is not intended to limit theinvention so further described in such appended claims.

1. A solid electrolytic capacitor comprising: an anode body formed froman electrically conductive powder, wherein the powder has a specificcharge of about 70,000 μF*V/g or more; a dielectric that overlies theanode body; and a solid electrolyte that overlies the dielectric,wherein the solid electrolyte includes a manganese oxide film that coatsat least a portion of the dielectric in a substantially uniform manner.2. The solid electrolytic capacitor of claim 1, wherein the electricallyconductive powder includes tantalum and the dielectric includes tantalumpentoxide.
 3. The solid electrolytic capacitor of claim 1, wherein thepowder has a specific charge of from about 120,000 μF*V/g to about250,000 μF*V/g.
 4. The solid electrolytic capacitor of claim 1, whereina carbon layer, silver layer, or both overlie the solid electrolyte. 5.The solid electrolytic capacitor of claim 1, wherein the capacitorexhibits an ESR of about 200 milliohms or less, at a frequency of 100kHz.
 6. The solid electrolytic capacitor of claim 1, wherein thecapacitor exhibits an ESR of about 100 milliohms or less, at a frequencyof 100 kHz.
 7. The solid electrolytic capacitor of claim 1, wherein thecapacitor exhibits a dry to wet capacitance percentage of about 80% ormore.
 8. The solid electrolytic capacitor of claim 1, wherein thecapacitor exhibits a dry to wet capacitance percentage of about 90% ormore.
 9. A method for forming a solid electrolytic capacitor, the methodcomprising: anodically oxidizing an anode body to form a dielectriccoating, wherein the anode body is formed from a powder; contacting thedielectric-coated anode body with a solution that contains a manganeseoxide precursor and a dispersant; and pyrolytically converting theprecursor to a manganese oxide solid electrolyte.
 10. The method ofclaim 9, wherein the dispersant contains an organic compound having ahydrophilic moiety and a hydrophobic moiety, which is an aromatic orheteroatomic ring system having from 6 to 14 carbon atoms.
 11. Themethod of claim 10, wherein the hydrophilic moiety includes a sulfonate,phosphonate, carboxylate, thiol, sulfonate ester, phosphite,phosphonite, phosphinite, phosphate, sulfate, phosphate ester,sulfoxide, sulfone, amino, or a combination thereof.
 12. The method ofclaim 10, wherein the organic compound has the following structure, or asalt thereof:

wherein, R₁ is an alkyl group having from 1 to 6 carbon atoms; R₂ is ahydrophilic moiety; m is from m to 8; p is from 1 to 8; and n is from 1to
 100. 13. The method of claim 10, wherein the compound is a salt of acondensed naphthalene sulfonic acid.
 14. The method of claim 10, whereinthe salt contains a sodium cation.
 15. The method of claim 10, whereinthe molecular weight of the compound is about 6,000 grams per mole orless.
 16. The method of claim 9, wherein the manganese oxide precursoris manganese nitrate.
 17. The method of claim 9, wherein the ratio ofthe surface tension of water (at 20° C.) to the surface tension of thedispersant (at a concentration of 1 wt. % in water and at 20° C.) isfrom about 0.5 to about 2.0.
 18. The method of claim 9, wherein theratio of the surface tension of water (at 20° C.) to the surface tensionof the dispersant (at a concentration of 1 wt. % in water and at 20° C.)is from about 0.8 to about 1.2.
 19. The method of claim 9, wherein thesurface tension of the dispersant (at a concentration of 1 wt. % inwater and at 20° C.) is from about 50 to about 95 dynes per centimeter.20. The method of claim 9, wherein the surface tension of the dispersant(at a concentration of 1 wt. % in water and at 20° C.) is from about 55to about 75 dynes per centimeter.
 21. The method of claim 9, wherein thesolution contains from about 0.001 wt. % to about 5 wt. % of thedispersant.
 22. The method of claim 9, wherein the solution containsfrom about 0.01 wt. % to about 1 wt % of the dispersant.
 23. The methodof claim 9, wherein the solution is an aqueous solution.
 24. The methodof claim 9, wherein the solution further comprises an alcohol.
 25. Themethod of claim 9, wherein the precursor is pyrolytically converted tothe oxide at a temperature of from about 150° C. to about 300° C. 26.The method of claim 9, wherein the solution is in the form of acolloidal suspension of nano-sized manganese oxide precursor particles.27. The method of claim 26, wherein the nano-sized particles have anaverage diameter of from about 0.1 to about 30 nanometers.
 28. Themethod of claim 26, wherein the nano-sized particles have an averagediameter of from about 0.2 to about 10 nanometers.
 29. The method ofclaim 26, wherein 90% or more of the particles have an average diameterof from about 0.2 to about 10 nanometers.